Nonaqueous electrolyte secondary batteries are a type of rechargeable battery in which ions move between the anode and cathode through a nonaqueous electrolyte. Nonaqueous electrolyte secondary batteries include lithium-ion, sodium-ion, and potassium-ion batters as well as other battery types.
Lithium-ion batteries are a popular type of nonaqueous electrolyte secondary battery in which lithium ions move between the cathode and the anode thought the electrolyte. The benefits and the challenges of lithium-ion batteries are exemplary of the benefits and challenges of other nonaqueous electrolyte secondary batteries; the following examples pertaining to lithium-ion batteries are illustrative and are not limiting. In lithium-ion batteries, the lithium ions move from the anode to the cathode during discharge and from the cathode to the anode when charging. Lithium-ion batteries are highly desirable energy sources due to their high energy density, high power, and long shelf life. Lithium-ion batteries are commonly used in consumer electronics and are currently one of the most popular types of battery for portable electronics because they have high energy-to-weight ratios, no memory effect, and a slow loss of charge when not in use. Lithium-ion batteries are growing in popularity for in a wide range of applications including automotive, military, and aerospace applications because of these advantages.
FIG. 1 is a cross section of a prior art lithium-ion battery. The battery 15 has a cathode current collector 10 on top of which a cathode 11 is assembled. The cathode current collector 10 is covered by a separator 12 over which an assembly of the anode current collector 13 and the anode 14 is placed. The separator 12 is filled with an electrolyte that can transport ions between the anode and the cathode. The current collectors 10, 13 are used to collect the electrical energy generated by the battery 15 and connect it to an outside device so that the outside device can be electrically powered and to carry electrical energy to the battery during recharging.
Anodes of nonaqueous electrolyte secondary batteries can be made from composite or monolithic anode materials. In composite anodes, particulate anode material is physically bound together with a binder forming a matrix of the particles and the binder. For example, anodes can be made from carbonaceous particles bound with a polymer binder. Monolithic anodes are anodes that are not made by the addition of a physical binder material. For example, any method of creating of a silicon anode where the silicon molecules are interconnected without the aid of an external binding agent is a monolithic film. Examples of monolithic anode materials include monocrystalline silicon, polycrystalline silicon and amorphous silicon. Monolithic anodes can also be formed by melting or sintering particles of anode material or by vacuum and chemical deposition.
During the charging process of the lithium-ion battery, the lithium leaves the cathode and travels through the electrolyte in the separator as a lithium ion and into the anode. During the discharge process, the lithium ion leaves the anode material, travels through the electrolyte in the separator and passes through to the cathode. Elements like aluminum, silicon, germanium and tin react with lithium ions and are used in high-capacity anodes. Anode materials that react with lithium have active areas in which lithium can react and inactive areas in which lithium cannot react. The ratio of the active to inactive area of the anode affects the efficiency of the battery.
In the reaction of lithium ions in a lithium-reactive material, there is a significant volume difference between the reacted and extracted states; the reacted state of lithium-reactive anode materials occupies significantly more volume than the extracted state. Therefore, the anode changes volume by a significant fraction during every charge-discharge cycle. In lithium-reactive anodes, cracks in the anode material are often formed during the cycling volume change. With repeated cycling, these cracks can propagate and cause parts of the anode material to separate from the anode. The separation of portions of the anode from cycling is known as exfoliation. Exfoliation causes a decrease in the amount of active anode material that is electrically connected to the current collector of the battery, thereby causing capacity loss.
Silicon anodes, which are excellent candidates for lithium-ion batteries due to silicon's high capacity for lithium, suffer from significant capacity degradation due to cycling exfoliation. Reducing the charged-to-discharged voltage window applied to a silicon anode in a lithium-ion battery can stem the capacity loss due to cycling since the expansion and contraction are a function of the state of charge. But reducing the charged-to-discharged voltage window lowers the operating capacity of the battery. Also, silicon is a poor conductor and must often be formulated with conductive additives to function as an anode material. These conductive additives reduce the active to inactive ratio, thereby reducing the energy density of the battery. Conductive additives are typically materials like carbon black that are added to the anode particles and mixed before binding the particles.
Another method to improve conductivity of an anode material is to deposit a layer of conductive material on an anode material. Methods for deposition of conductive layers include vapor deposition, electro-deposition, and electroless deposition. When materials are deposited using any of the above methodologies on resistive substrates like silicon, the deposition across the anode is typically non-uniform. For example, in electroless plating and electroplating of metals such as nickel, the deposition rate on a nickel surface is significantly higher than that on a dissimilar surface such as silicon. A deposition that has such significant kinetic variations on different materials causes the deposition to have surface defects, pores, and areas of no deposition. In the case of a line of sight deposition processes like vacuum deposition from a target, non-planar surfaces with areas that are not in direct line of sight get significantly less or no deposition thereby reducing thickness uniformity. In addition, these coatings may not adhere well since these coating methods have poor adhesion strength of the deposited metal to semiconductor material. The poor adhesion strength, poor uniformity, and poor minimum thickness of these coatings result in poor cycle life, power, energy, and reliability.